Amplification and modulation



0d. 28, 1941. A 5`ENAUKE v MPLIFICATION AND MODULATION Filed Mrch 3o, 1958- A 4 Sheets-Sheet l i 1 I I :1

l l Zero Power Level nodumion grad Bias voli'ag l af tube l.l

\ annif, bias.

Pla?. currentI `tube N.

Plaie current Tube l.

Oct. 28, 1941. A. sENAUKE I AMPLIFICATION AND MODULATION Filed March 30, 1938 4 Sheets-Sheet 2 Pcak posltlve modulaflon.

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Power oufpuf Jmbe'N.

Carrier level.

Carrier plaie .curranf pulse.

To anfenna P,

To anfenna.

, INVENI'OR A LEXA/vaine .SZW/:UKE

ATTORNEY Oct. 28, 1941. I A. SENAUKEl MPLIICATICN AND MODULATION Filed March 30, 1958 4 Sheets-Sheet 3 Tomate o f 22 plafe of!! Oct. 28, 1941. A; -sENAuKE AMPLIFICATION AND MODULATION 4 Sheets-Sheet '4 Filed March 30, 1958 HoHulafed Modula'led INVENTOR' Aa-'xA/vfe Sama/KE ATTORNEY Patented Oct. 28, 1941 AMPLIFICATION AND MODULATION Alexander Senauke, New York, N. Y., assigner to Amperex Electronic Products, Inc., Brooklyn, N. Y., a corporation of New York Application March so. 193s, serial No. masas (ci. om 11 Claims.

This invention relates to methods and apparatus for amplifying and modulating electric waves and for amplifying already modulated waves such as are employed in radio and carrier current communication systems.

Among the objects of my invention are the following:

To provide ducing so-called grid-bias modulation and amplification operating at relatively high plate elllciencies, of the order of 60 to 80%;

To provide apparatus for producingI linear power amplification of a completely or 100% modulated super-audible frequency oscillation, operating with relatively high plate eillciency, of the order oi 60 to'l0%;

To provide a method and apparatus 'for driving an oscillatory circuit with a completely (100%) modulated oscillation of its own frequency, in which the tubes are so arranged and operated as to permit the utilization of tubes of considerably lower power rating for a given load'than has heretofore been the case;

To provide circuits employing the tubes for the purposes stated in a manner that'assures the desired high levels oi emciency, and prevents the development of conditions which would tend to impair the quality of the output or lower the efiiciency obtained;

To provide means for driving a common load circuit by means of a plurality of tubes or groups of tubes, one tube or group of tubes only furnishing the driving power when the output required is less than a predetermined value, and, the other tube or groups of tubes furnishing power only when the demand exceeds a predetermined value, the transition being accomplished automatically under the control of the waves or waves impressed on the input of the system;

To provide means whereby a plurality of tubes or groups oi tubes requiring different impedances may supply a common load by tapping said load at proper points, or by coupling to said load by diiierent couplings, or by adjustment of the tube plate voltages; and

To provide certain novel methods of connecting and employing tubes to produce the results mentioned.

Still other objects will be apparent from the specification.

In this application I have particularly pointed out and distinctly claimed 'the part, improvea method and apparatus for pro- Fig. 1a being :a diagrammatic showing of a A100% modulated carrier wave,

Fig. 1b,l showing the variation in grid'bias on tube N,

ilsv

Fig. 1c the variation in grid bias on tube P,

. Fig. 1d showing plate current of tube N, the portion of this curve covering the time when tube P is acting being omitted from 1d and being reproduced on a larger scale in Fig. 1f for purposes of clarity,

Fig. 1e showing the plate current in tube N.

Fig. 1f showing, on an enlarged scale, the plate current of tube N in a portion of the interval left blank in Fig. 1d, and

Fig. 1g being power output curves of tubes N and P plotted on the same axes, all of the curves being qualitatively, but not quantitatively, plotted for the portion of the carrier wave shown in Fig. la;

Fig. 2 is a circuitdiagram of a grid-bias modulated ampliier according to one form of my invention, the apparatus being shown in its most complete form to aid in the understanding of my invention; j

Figs. `3 and 4 arev circuit diagrams of alternative forms of apparatus o! the type of that shown in Fig. 2;

Fig. 5 is' a circuit diagram of a linear power amplier for the amplification of a modulated carrier oscillation;

Figs. 6 and 'l are circuit diagrams of alternative forms oi apparatus of the type of that shown in Fig. 5;

Fig. 8 is an alternative form of circuit which .tubes or groups of tubes, one (the N tube) supplying the major portion of power during negative modulation, the other (the P tube) supplying the power during positive modulation, this ment -or combination which I claim as my inbeing accomplished by providing that the tube I or tubes active in negative modulation deliver tive modulation, this power is always only a small partv of the total power'during positive modulation, and, generally speaking, the actual amount of power delivered by the N tubes decreases with increase in positive modulation, as shown in Fig. 1g.

' nection providing for energy from tube P to reach Filiilteferring now more particularly to Figs. 1 and Y 2, Fig. 1a represents a small time sectionl of a carrier wave, varying in amplitude within the envelope b--b, representing a lower frequency or frequencies,usually sound; dotted lines c-'c represent the constant amplitude of the carrier when no modulation is being supplied, as whenv no sound is impinging on4 the microphone; dotand-dash lines d-d represent the maximum amplitude of the carrier (by definition of 100% modulation, the maximum amplitude d is twice that of amplitude c).

As will' be understood, the amplitude of the carrier, when modulated, may be rapidly varyingy between zero and 'amplitude d, in a very complicated manner, determined by the modulating frequencies. When the carrier amplitude exceeds the limit c, the modulation is said to be positive; when less than c, negative. AThese terms will be used herein with those meanings.r

Also, these tubes or groups of tubes may feed a common load circuit, and usually the tubes will require different impedances. I have provided various arrangements by which the load impedances may be properly related to the tubes.

This invention -diilfers from other proposed schemes for obtaining high efilciency grid modulation, in which the plate voltage and static bias are made to expand partially at a syllabic rate to allow of positive modulation, or for providing high efficiency linear amplification by the same means or by means of a variable load impedance which is made to decrease in effective value for positive modulation requirements by the action of an auxiliary tube or tubes.

Referring first to Fig. 2, the carrier frequency oscillation, which may at this point have a relatively low power, is supplied from any suitable driving source to a coil I, which may be coupled to coil 2 shunted by condenser 8, the same form-` ing a tuned input circuit commonto tubes N and lP, the tube N being the negative modulation to the grid 9 of the tube P. The plate 5 of tube N may be connected to one terminal oi inductance I2 shunted by a variable condenser I6, the said inductance I2 and condenser I3 forming -the output tank circuit. The other terminal of inductance I2 may be connected to the positive terminal of a source of space current for energizing the tubes N and P, which source is omitted for simplicity, and one side of the tank circuit may be grounded through condenser I4, as shown.

Coupled to inductance I2 I may provide inductance I5, which may be connected to any suitable load circuit, such, for instance, as an antenna and ground (not shown). 'I'he plate 8 of tube P may lbe connected to the mid-point (or a variable point) of inductance I2, said conthe tank circuit and serving also to connect the plate 8 to the source of plate current B. This connection of the plates of tubes N and P at different points on the tank circuits allows of properly relating the impedance of the tank circuit as seen from the plate, to the respective tubes N and P which may require different load impedances.

'I'he cathodes 4 and 1 may be heated in lany suitable manner (not shown), and the opposite conductors feeding the lament may be grounded through condensers I1 and plate capacity, it may be desirable to provide means for preventingv oscillation thereof (since the grid and plate circuits are preferably tuned to the same frequency). and this may be readily accomplished by twin neutralizing condenser consisting of plates I9 and 20 respectively connected to the plates of tubes P and N and having a common cooperating plate element 2l connected to the low side of inductance 2, the midpoint of which may be grounded.

The audio signals with which it is desired to modulate the carrier may be supplied from an'y f and P respectively to control the operation of tube, the tube P the positive modulation tube.

Each of the tubes is-herein" indicated as a triode, with which very satisfactory results have been obtained, although it will be understood that other types of tubes may be employed, and that a group of tubes in parallel may be employed both for N and P tubes.

Tube N may comprise cathode 4, anode 5 and control electrode or grid 6. Similarly tube P may comprise cathode 1, anode 8 and control electrode 9.

Froml the tuned circuitformed by inductance 2 and condenser l, a connection may be made 3 to the grid of the tube N through condenser I8,

and a similar connection through condenser II the apparatus. a

One way in which this may be accomplished is by connecting in series acros'sthe secondary winding 23 a rectiler 25, which may be of the Fleming valve type, having a cathode 25a and an anode 25h, in series with resistance 26. I may likewise connect across the secondary winding 24 a second rectifier 21 having a cathode 21a and an anode 2lb, the said rectifier being in series with resistance 28.

It will b e understood that, since current flows in one direction only through these rectifiers, whenever an audio frequency current is impressed on winding 22, current will flow through the resistances 26 and 28, each of these currents being a pulsating uni-directional current, and causes a corresponding uni-directional voltage IIR drop) across the resistances. From the negative terminal of the resistance 26 We may provide a connection to the grid 6 of tube N through radio frequency choke 29 and a return thereof to the cathode throughvariable tap 30 engaging resistance 26 and connecting to asuitable point on the battery B', the said battery serving to provide fixed bias for the grids of` the tubes P and N (the term "fixed being used in the sense thatthe bias voltage is not controlled by the sig- I8 respectively. Since the tubes shown may have substantial gridascuas nal, but, of course, it will be adjusted to the desired value) To provide the bias on tube P the negative side of resistance 2B may be connected to the positive terminal of the battery B' and a variable tap on resistance 28 connected to the grid of tube P through radio frequency choke The magnitudes of the radio frequency grid input voltage and the respective fixed grid biasing voltages for tubes N and P are so related that when the audio frequency voltage is zero,

tube N is delivering the required carrier power to the common load circuit with 'a high plate efficiency and tube P is delivering zero or negligible power. This condition will obtain if the xed biasing voltage for tube N is lower than the peak value of the radio frequency grid voltage and the fixed biasing voltage for tube P is equal to or sumciently larger than the peak value of the radio frequency grid voltage to prevent any appreciable plate current iiow.

The low frequency bias voltage which produces]y the modulation will'be seen to be supplied in series with the fixed biasing voltage and it 'will I the bias on tube P. This will be clear from Figs.

1b andlc.

Tracing the bias circuit on tube N itwill be observed that the grid 6 is negative with respect to the cathode by the amounty of the potential of battery B included and that .the grid is made still more negative with respect to the cathode by the amount of the drop through resistance 26, if any. Thus, as the audio frequency current increases in negative modulation, the grid 6 will go still more negative with respect to its cathode and the space current of tube N will-be decreased.

On the other hand, tracing the bias circuit for tube P, it will be seen that while the grid tends to be made negative with respect to its cathode by the amount of the potential of battery B', the voltage drop through resistance 28 is applied in the opposite direction so that with increase in audio frequency current for positive modulation, the potential of the grid 9 moves in a positive direction with respect to its cathode.

Audio currents which cause negative modulation cannot change the bias on tube P, and audio currents which cause positive modulation cannot change the bias on tube N, because rectiiiers 25 and 21 pass current only in the direction corresponding to negative modulation and positive modulation respectively.

The magnitude of the audio frequency voltage should be so related to the magnitudes of the fixed biasing voltage, the constant radio frequency grid voltage (the actual amount of which, supplied to tubes N and P, may be controlled by taps (not shown) from condensers l0 and Il to variable points on coil 2), the effective plate load impedance provided by the common tank or load circuit, and the characteristics of the tube, that at the maximum peak audio voltage to be supplied, the power output of tube P will approach four times the carrier power supplied by tube N under static bias conditions (no sound input). Under this positivev peak condition the power delivered to the tank circuit is the value required to make the peak radio frequency voltage across the entire tank circuit twice its carrier (zero modulation) value. Under the conditions of high efficiency adjustment. this value of the peak radio frequency voltage will exceed the direct current plate voltage on tube N by values up to of the direct current plate.

voltage on tube N, and when the modulation is positive and at the instant when the radio frequency grid driving voltage is at its maximum positive value, the polarity of the radio frequency tank voltage will be opposite that of the direct current plate supply voltage of `tube N and the effective plate-filament voltage of tube N will be negative, and may be as much as 80% of the battery voltage in magnitude. The point between carrier amplitude c and peak amplitude at which the plate of tube N goes negative with respect to its cathode and the tube ceases supplyingr power completely, will be determined by the choice of the various variable factors.

I use the terms positive audio and "negative audio in the same sense as positive and negative modulation; that is to say, audio currents which cause positive modulation (greater than unmodulated carrier amplitude) are called "positive audio, those which cause negative modulation (less than unmodulated carrier level) are called "negative audio."

For positive audio conditions, the power delivered by tube P must be so proportioned as to make the total power delivered by both tubesl N and P vary as the square of the ratioof the instantaneous radio voltage to the maximum total effective bias to a value suflicie'ntly greater than the peak radio frequency grid voltage to reduce the power output of the tube N to zero (see Figs. la, 1b and 1d). Between the limits of zero output at the "maximum negative audio peak" and zero audio (no sound input), the power delivered by tube N must also vary in accordance with met) A with El negative. Thi-s condition is also readily obtained'in practice. A

This circuit combination affords a very high operating eillciency during carrier conditions (no modulation) and a high average efilciency while the output is modulated, yet requires a negligible amount of audio power for modulating purposes. As compared with conventional modulated amplifiers it may be considered as offering the operating emciency of a plate modulated amplifier, with the minimized audio power requirements of the conventional grid bias'modulated amplifier (which has only half the operating efciency). Practically, tube vP may have a rather low average power handling capacity, though its peak power handling ability must be equal to that of a plate modulated tube of corresponding carrier output rating. Tube N may have peak power handling capacity no greater than the required vcarrier power and it need have a radio frequency voltage tolerance only approximately 75% of that required of similartubes carrying an equal carrier rating for plate modulation use.

In -specially designed tubes, these requirements may be met with a total tube investment only slightly greater than the cost of the radio frequency tubes only of a plate modulated amplier. With conventional tubes the cost may be double that of the radio frequency tubes only of a plate modulated radio frequency amplifier, which still will make the cost only about half of that of a conventional linear or grid-bias modulated amplifier of the same capacity.

By suitable choice of tubes N and P, especially as regards their respective amplification constants and mutual conductances, as for instance by using a low a tube for tube N and a high a tube for tube P, the bias and audio modulation requirements may be simplified as follows:

As shown in Fig. 3, it will be noted in this instance that two secondaries are no longer employed, the primary 22 being provided with a single secondary 33, `the rectifiers 25 and 21 and resistances 26 and 28 in series with the respective rectifiers being connected directly across the winding 33, but in opposite directions. The operation of the circuit in principle, aside from these changes, is the same as that described with 1 reference to Fig-2 and, it is thought, need'not be repeated.

If desired, tubes N and P may be operated with different plate voltages, as indicated in Fig. 4, the plate of tube P being supplied through radio frequency choke 35 and the output current being fed through condenser 36 to the tank circuit and the plate of tube N being operated at a lower potential through radio frequency choke 34 and its output fed to the tank circuit at the same point at which tube P feeds the tank ciri cuit, and this common point feed may be employed with any of the circuits shown, provided the tube impedances are propelry related by choice of the proper plate voltages.

While I have shown the rectiiiers 25 and 21 for simplicity in explaining the action of the system, it should be understood that the same are not actually necessary and may be omitted,

together with resistances 26 and 28, tap 30 then connecting to the upper terminal of secondary 23 in Fig. 2, and the grid return of tube P being connected to the upper terminal of secondary 24.

Similarly, in Fig'. 3 the grid returns of tubes i N and P may be brought directly to the upper terminal of secondary 33 and rectlfiers 25 and 21 and resistances 26 and 28 omitted.

The reason for this is that the simultaneous self-rectification of tubes N and P themselves may serve the purpose of rectifiers 25 and 21. On positive modulation, an increase in the posii tive swing of the'grid of tube N has no effect, because at carrier level all the current that tube can pass is fiowing through the plate circuit, and

it is only necessary to protect the tube against excessive grid currentr flow. Various means by which this may be done are shown and explained hereafter.

For negative modulation/the negative voltage y output circuits connected both on the same side of ground, they may be connected on opposite side of an intermediate ground and plate circuit supply connection on the input and output circuits respectively.

Referring now to Fig. 5, the principles of my inventions are shown in that figure as applied to a high efllciency linear amplifier, arranged to provide power amplification forv a modulated radio frequency wave applied to the input circuit. The circuit herein shown is essentially, insofar as the operation of tubes N and P and the circuit arrangement pertaining thereto is concerned, the same as that in the previous figures, With the exception-that in this instance biasingvoltages are not generated by the action of separate rectifiers. In this instance it will be noted that the bias on the grid 6 of tube N is provided through radio frequency choke 29 and self-biasing resistor and an initial fixed biasing source of potential 4|. The bias forv tube P may be provided through radio frequency choke 3l and fixed biasing source of potential 42. The modulated radio frequency wave may be supplied through input coil I and through the tuned circuit consisting, as before, of inductance 2 and condenser 3.

The tube N in this instance is so adjusted, as l to its static or fixed bias potentiaLand effective plate load resistance (which may be controlled by adjustment or choice of the turns of coil I2 in the tank circuit) so as to operate as a class B amplifier with peak elciency under carrier coil l2 so that under peak or maximum positive modulation conditions it will function as a class C amplifler of high efllciency to deliver to the tank circuit the additional power required to double the total radio frequency tank circuit voltage.

The tube N should be provided with means to prevent a destructive rise in its grid power loss during positive modulation periods. This may be accomplished in a number of ways. For instance, this tube may be of a low u type so operated that under carrier conditions, i. e., neither negative nor positive modulation, its grid current will be relatively low or zero, i. e., the instantaneous peak grid voltage being just equal to,

or only slightly greater than the static bias voltage. Under these conditionsits static direct current bias may be supplied through a very high during positive modulation so that the grid starts running positive and drawing current, this current flow through resistance 40 will produce a seit-biasing voltage tending to limit the grid current. On negative modulation the tube P will not function since it will be remembered that a radio frequency driving voltage equal to the unmodulated carrier voltage is required to overcome its cut-oil bias.

Another-Way of providing a protecting overbias for tube N for positive modulation is shown in Fig. 6, which differs from Fig. in that the rising space current of the tube P is employed to furnish the protecting over-bias for the tube N. In this instance, resistance 48 is shown as connected in the space current path of tube P, with a connection through biasing source I8 and through radio frequency choke tothe grid of tube N.

As the cut-oil bias of. tube P is overcome when positive modulation is applied, the increase in space current flowing through resistance 45 produces an IR drop tending to make the lower end of resistance 45 more negative than the upper end, and this voltage is applied to the grid of tube N. It should be noted that in this case the resistance 45 should be relatively small so as not to appreciably aifect the operation of tube N on negative modulation. It will be noted that this arrangement requires a separate plate supply for thetubes N and P.V

Still another way of providing the over-bias for the tube N is indicated in the circuit of Fig. '7,

wherein an inductance 5D is shown coupled to the tank circuitto be excited by the current owing therein. In circuit with the coil 50 there may be provided a rectifier 5|, which may be of the Fleming valve type, having cathode l2 and anode 53, and with its circuit completed through resistance M and source of potential 55. The grid return in tube N in this instance after passing through radio frequency choke 29 may be taken to a variable tap on the source 55. When the tank circuit begins to draw current, a voltage is produced in coil 50. When this voltage is great enough to overcome the voltage of battery (which is so poled as to oppose said voltage), rectier 5l begins to draw current. This produces a voltage drop through resistance 54 of the polarity indicated, and tending to make the grid of tube N more negative with respect to its cathode. By adjustment of the various values, this bias, which increases negatively with increase in tank circuit current, may be made to protect tube N against excessive grid current.

Still another way 'of protecting tube N against excessive grid current is shown in Fig. 8. In this instance I may interpose in the grid return an audio frequency choke 6U, shunted by rectier 6l, which may be of the Fleming valve 1lfl'lJe.'

Wh'en the grid is swung positive, as long as no substantial grid current flows, the plate of rectier 8B is positive with respect to its cathode, and it forms in effect a short-circuit around choke Ell, permitting negative modulation.

On positive modulation, as the grid begins to draw current, that current passing through choke 80 makes the plate of rectifier 8l nega tive with respect to its nlament, terminatina the flow of current therein, and choke 80 acts as a self-biasing impedance protecting tube N against excessive grid current.

and P as energizing a `common load circuit or' tankl circuit, it may be pointed out that my invention may be beneficially employed when the receiver itself is the common load circuit; that is to say, the tube N may feed a tank circuit and -an antenna so that the antenna radiates only in response to operation of tube N, as shown in Iiig. 9. Asecond antenna may be provided with its associated tank circuit driven by tube P. It is desirable in this case. however, that the two antennas which, for convenience, may be termed the N antenna and the P antenna, are so placed and arranged lthat insofar as reception is concerned the receiver is actuated by both antennas equally. If this condition is fulfilled the received signal will then be identical with a signal received from a single antenna Y lated to the tubes by choice and adjustment ofI the various constants, including the coupling between the respective tank coils and the load coils LN and LP. l

While I have shown and described certain preferred embodiments of my invention, it will be understood that modifications and changes may be made without departing from the spirit and scope of my invention, as will be understood by those skilled in the art.

I claim:

l. In a system for supplying amplified superaudible frequency, electric waves modulated at a lower rate, in combination, a plurality `oi vacuum tubes having their input circuits fed in phase from a common source, said tubes having their output circuits coupled together, a load circuit fed by said tubes, a' tank circuit interposed between said load circuit and said tubes, an energy transfer path from one of said tubes to-said tank circuit for driving the same, a second energy transfer path from another of said tubes to said tank circuit, said paths being connected to said tank circuit at points where the radio frequency voltages in said tank circuit are different and forming a voltage transfer path between said tubes impressing on said second tube from said tank circuit a voltage in excess of that impressed on said-tank circuit by said first tube. v

.2. In asystem for supplying'amplifled superaudible frequency electric waves modulated at a lower rate, in combination, a plurality of vacuum tubes having their input circuits fed in phase from a common source, a load. circuit fed by said tubes, a tank circuit interposed between said load circuit and said tubes, a circuit from one of said tubes, said circuit including'a portion of said tank circuit, a second circuit from another of said tubes, said second circuit including a portion of said tank circuit, said cir-- cuits including unequal impedances in said tank In this connection it should be understood lower rate, in combination, a plurality of vacuum tubes having their input circuits fed in phase from a common source, said tubes-having their output circuits coupled together, a load circuit fed by said tubes, means for causing part but not all of said tubes to deliver the maior p0rtion of output power to said load circuits during negative modulation, and means for causing the balance of said tubes to deliver the maior portion of power to said load circuit during positive modulation, said balance of said tubes being connected to the first mentioned part of said tubes through a voltage step-up path to render the plate of the iirst mentioned p'art of said tubes negative with respect to the cathode (tE-itu p where E1 is negative, and for causing a second portion of said tubes to deliver power, during at least a part of the periodof positive modulation, substantially proportional to E,- 3 (TF1) where E1 is positive, said means comprising a voltage path between the plate elements of the tubes of the iirst and second portions, said path providing a voltage step-up in the direction from the second to the iirst portion, El represents the instantaneous radio frequency voltage and Em is the maximum peak radio frequency voltage. l

5. In a system for supplying amplified superaudible electric waves modulated at a lower rate, in combination, a plurality ot vacuum tubes having their input circuits fed in phase from a common source, said tubes having their output circuits coupled together, a load circuit fed by said tubes, means for causing part only of said tubes to deliver power, during negative modulation, substantially proportional to yWhere El is negative, and for distributing the load between said tubes so that on positive modulation the delivered power is always substantially proportional to where E1 is positive, said means comprising' means for rendering one of said tubes inoperative during negative modulation, means for increasing the bias in a negative direction on a second tube during negative modulation, means for rendering ythe rst tube operative during positive modulation, and a voltage step-up path accesso 6. In a system for supplying ampliiied superaudible electric waves modulated at a lower rate, in combination, a plurality of vacuum tubes havl ing their'input circuitsl fed in phasev from a. common source, said tubes having their output circuits coupled togethe a load circuit fed by said tubes, means for causing a first part only of said tubes to deliver. power, during negative modulation, substantially proportional to Pomllal t0 where El ispositive, when modulation is Dositive, and controlled by the amount of power to be delivered, said first mentioned tube delivering only a minor portion of the power output when modulation is positive, said means comprising means for applying a bias to said first part of said tubes, means for increasing said bias negatively for negative modulation, plying a cut-oil? bias to another portion of said tubes during negative modulation and means for applying a cut-oil voltage to the plate of the rst part of said tubes during each cycle in peak positive modulation, Ei represents the instantaneous radio frequency voltage and Em is the maximum peak radio frequency voltage.

7. In a system for supplying amplied superaudible frequency electric waves modulated at a f. lower rate, in combination, a plurality of tubes having a common input circuit delivering input voltage in the same phase to said tubes, said tubes requiring diierent' impedances, a common Power driven circuit associated with said tubes to be driven thereby, and connections from said tubes to different points of said driven circuit for properly vrelating the impedance of said circuit as seen from the plates of said tubes.

8. In a system for supplying amplified superaudible frequency electric waves modulated at a lower rate, in combination, a plurality of tubes having a common input circuit delivering input voltage in the same phase to said tubes, said tubes requiringdiiferent impedances, a power driven circuit associated with one of said tubes to be driven thereby and having substantially the impedance required by said tube, and means for coupling said driven circuit to another of said tubes and for reducing the impedance of said circuit as seen from the plate of said tube.

9. In a system for delivering electric waves to a load circuit, in combination, a pair of vacuum tubes havingl a common input circuit delivering input voltage in the` same phase to said tubes, said tubes requiring diierent plate load impedances in order to deliver unequal relative proportons of plate power output, a common output circuit fed by said tubes, andmeans for relating the impedance of said load circuit to said tubes in such'manner as to obtain the required impedances.

10. In a, system for supplying amplified superaudible frequency electric waves modulated at a lower rate, in combination, an input circuit delivering input voltage in the same phase to said tubes, a plurality of tubes supplied in phase from said input circuit, said tubes having their platecircuits coupled to each other, anA output circuit to -be supplied by said tubes, said tubes having different power delivery capabilities, and conmeans for apcathode when. the second tube begins to l0 plate current.

ulation, and means for causing said tubes to deliver less power for negative modulation, means for biasing another of said tubes to cut oil at or about zero modulation, whereby on positive modulation said tube delivers power, and connections between the plate circuits of said tubes for causing the plate voltage of the rst tube to move in a negative direction with respect to its pass ALEXANDER SENAUKE. 

